1. Field of the Invention
The invention relates in general to a preamplifier for a receiver and a method thereof, and more particularly to a preamplifier with wide range of the common voltage of the input differential voltage pair.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a conventional rail-to-rail preamplifier for a receiver. The conventional rail-to-rail preamplifier 100 includes amplifiers 110 and 120 and an inverter 130. The amplifiers 110 and 120 each amplify a differential voltage pair VN and VP and produce one voltage of the amplified differential voltage pair for the inverter 130. The inverter 130 then pulls its output voltage Vo high or low based on these inputs. The transistors in amplifiers 110 and 120 are complementary, so that the preamplifier 100 is capable of amplifying the differential voltage VN and VP with wide common voltage range.
However, the amplifiers 110 and 120 are powered by a high voltage supply power HVDD, which is for analog power, while the inverter 130 is powered by a low voltage supply power LVDD, which is for digital power. The high supply voltage HVDD is around 3.3V, and the low supply voltage LVDD is around 1.8V. The low supply voltage LVDD is even lower than the high supply voltage HVDD minus a threshold voltage of a transistor 121 in amplifier 120. Thus, the transistor 131 in the inverter 130 is cut off, which causes the rail-to-rail preamplifier to become disabled.